Hand bearing compasses are commonly used in navigation to take bearings of known visible marks in order to obtain a fix of position. The conventional hand bearing compass has a liquid damped compass card arrangement provided with a diametrical sight-line for alignment with the chosen distant mark. The body of the compass is usually adapted to be held in one hand, commonly having an elongate handle projecting axially downwardly beneath the card which makes the compass bulky in three dimensions and consequently inconvenient for storage and portability. To keep the card horizontal it is pendulously mounted on a central pivot, but this has the consequence that lateral acceleration caused by sea motion will cause rotary oscillation of the card. This makes the instrument difficult to read at sea.
Earlier U.S. Pat. No. 4,590,679, assigned to the same assignee as the present application, describes an electronic magnetic fluxgate that can be used as a direction finder. Such a sensor still needs to be held horizontally, because the earth's magnetic field has inclination and measurement of any component other than the horizontal will produce inconsistencies. If the sensor were provided with a gimbal system to keep it horizontal the gimbal would be disturbed by lateral accelerations caused by sea motion and errors would be introduced. Gimbal errors have therefore to be eliminated by fixing the magnetic fluxgate relative to the instrument body and finding some means of aligning the instrument body by hand in the horizontal plane.